Francisco J. Azcona

Dealing With Speckle Effects in Self-Mixing Interferometry Measurements

An analysis of speckle effects and the techniques to overcome them in self-mixing interferometry signals is presented. We characterize the effect of surface roughness and laser spot size on the speckle modulation of the signal, and then propose two simple experimental approaches to overcome the amplitude fading induced by the speckle effect. Unlike the techniques proposed until now, our first approach uses an adaptive optical element in the form of a voltage-programmable liquid lens, which adaptively changes its focal length to modify the speckle pattern. Our second approach combines two laser signals which present different performance parameters. By using any of these simple methods, the introduction of inaccuracies in the measurement process due to speckle is avoided.

A Nanometric Displacement Measurement System Using Differential Optical Feedback Interferometry

We propose differential optical feedback interferometry, a technique able to measure nanometer-size amplitude displacements by comparing the optical power of two lasers subject to optical feedback. In this letter, the principles of the technique are explained in detail, and its limits are explored by simulation. Theoretical results are presented showing that the technique can measure nanometer scale displacements with resolution within the angstrom scale. An experimental setup for validation has been built, and a series of experimental tests were performed using a capacitive sensor as a reference. Results show good agreement between theory and experiment with a reasonable reduction in performance due to mechanical coupling and signal noise. The proposed technique, thus, provides measurements of a very high resolution using an extremely simple and robust experimental setup.

Analysis and control of speckle effects in self-mixing interferometry

An analysis of speckle effects in self-mixing interferometry signals has been performed. We will characterize the effect of surface roughness and laser spot size on the speckle modulation of the signal, and we will propose two simple experimental approaches to overcome the amplitude fading induced by speckle effect. Differently to the techniques proposed up to the moment, our first approach uses an adaptive optical element in the form of a voltage programmable liquid lens, which adaptively changes its focal length to modify the speckle pattern. Our second approach combines two laser signals which present different performance parameters. By using any of these simple methods, the introduction of inaccuracies in the measurement process due to speckle is avoided.

Current Developments on Optical Feedback Interferometry as an All-Optical Sensor for Biomedical Applications

Optical feedback interferometry (OFI) sensors are experiencing a consistent increase in their applications to biosensing due to their contactless nature, low cost and compactness, features that fit very well with current biophotonics research and market trends. The present paper is a review of the work in progress at UPC-CD6 and LAAS-CNRS related to the application of OFI to different aspects of biosensing, both in vivo and ex vivo. This work is intended to present the variety of opportunities and potential applications related to OFI that are available in the field. The activities presented are divided into two main sensing strategies: The measurement of optical path changes and the monitoring of flows, which correspond to sensing strategies linked to the reconstruction of changes of amplitude from the interferometric signal, and to classical Doppler frequency measurements, respectively. For optical path change measurements, measurements of transient pulses, usual in biosensing, together with the measurement of large displacements applied to designing palliative care instrumentation for Parkinson disease are discussed. Regarding the Doppler-based approach, progress in flow-related signal processing and applications in real-time monitoring of non-steady flows, human blood flow monitoring and OFI pressure myograph sensing will be presented. In all cases, experimental setups are discussed and results presented, showing the versatility of the technique. The described applications show the wide capabilities in biosensing of the OFI sensor, showing it as an enabler of low-cost, all-optical, high accuracy biomedical applications.

Extraction of vibration parameters from optical feedback interferometry signals using wavelets

This paper proposes the use of the wavelet transform as a technique that is suited for fringe detection and analysis of optical feedback interferometry (OFI) signals, thus allowing the retrieval of extremely small physical motion phenomena. A novel algorithm based on wavelet transform is used to process the OFI signal simultaneously in the time and frequency domains, enabling precise detection of signal fringes and, thus, the extraction of amplitude features of the vibrating target with error in the order of 0.1λ. Furthermore, using a complex Morlet wavelet as an analyzing wavelet enables us to extract important information from the time envelope of the OFI signal. Such an envelope can be useful in detecting fringes even in the presence of speckle reducing the error in a displacement reconstruction. Examples of OFI applications, including nanometric displacement sensing without direction ambiguity as well as the measurement of the frequency and velocity of vibrating targets and the detection in time of no periodic events, are also presented using this wavelet approach.

Frequency-Modulated Optical Feedback Interferometry for Nanometric Scale Vibrometry

We demonstrate a novel method that makes an efficient use of laser nonlinear dynamics when subject to optical self-injection for subwavelength displacement sensing purposes. The proposed methodology combines two different phenomena taking place inside the laser cavity: optical self-injection, which results in optical feedback interference, and laser continuous wave frequency modulation, giving rise to a wavelength sweeping effect in the lasers emission. We present a combination of these phenomena to measure vibration amplitudes below λ/2 with the resolutions of a few nanometers, bandwidth dependent upon the distance of external target, amplitude, and frequency of current modulation. The basic theoretical details and a mathematical model are presented for the developed measurement principle. Experimental results with the system working as a vibrometer to measure a target vibration of amplitude λ/5 (137.5 nm) with a mean peak-to-peak error of 2.4 nm just by pointing the laser diode onto the target and applying some signal processing are also demonstrated.

Differential self-mixing interferometry for micro-cantilever motion sensing

Force sensing is a common practice used for the characterization of matter properties and in particular of bio-materials. Different optical methods have been used in the past to allow high resolution force measurements while avoiding uncertainties induced by external loading of contact sensors. In this paper, we propose the use of differential self-mixing interferometry, a self-aligned, cost effective and compact technique that allows the measurement of displacements with a theoretical resolution in the order of λ/2000 and a practical resolution in the order of λ/200 in practical applications. The DSMI sensor is used to detect the motion of a rectangular cross section cantilever placed on a piezoelectric stage. The measurements were compared with the signal received from the internal piezo stage capacitive sensor, which has a nominal resolution of 2nm. Results show that the DSMI sensor is able to follow accurately the cantilever displacement. A discussion of the potentials, limitations and required further developments of the method will also be presented.

Extracting vibrational parameters from the time-frequency map of a self mixing signal: An approach based on wavelet analysis

The characterization of the time-frequency map that is obtained from wavelet analysis allows us to view the temporal and spectral components of non-stationary signals simultaneously. Wavelets thus provide a complete platform from where the parameters of interest in the signal in time and/or frequency domain can be extracted. In this paper, we analyse the conventional self mixing interferometry signal (SMS) in the time-frequency domain by using wavelet analysis, in order to extract vibrational features of the target directly from the SMS, to detect the moment in time when the direction of the target changes, the velocity of the target or its displacement.

A method to measure sub nanometric amplitude displacements based on optical feedback interferometry

Optical feedback interferometry is a well known technique that can be used to build non-contact, cost effective, high resolution sensors. In the case of displacement measurement, different research groups have shown interest in increasing the resolution of the sensors based on this type of interferometry. Such efforts have shown that it is possible to reach better resolutions by introducing external elements such as electro-optic modulators, or by using complex signal processing algorithms. Even though the resolution of the technique has been increased, it is still not possible to characterize displacements with total amplitudes under λ/2. In this work, we propose a technique capable of measuring true nanometre amplitude displacements based on optical feedback interferometry. The system is composed by two laser diodes which are calibrated within the moderate feedback regime. Both lasers are subjected to a vibration reference and only one of them is aimed to the measurement target. The optical output power signals obtained from the lasers are spatially compared and the displacement information is retrieved. The theory and simulations described further on show that sub-nanometre resolution may be reached for displacements with amplitudes lower than λ/2. Expected limitations due to the measurement environment will also be discussed in this paper.

Optical feedback interferometry for measuring dynamic stress deformation of beams

An optical feedback interferometer has been used as a sensor for measuring the deformation of the beams under dynamic loading. The compactness, non-contact nature, high accuracy (below half wavelength of the laser) and the cost-effectiveness of this sensor makes it a suitable choice for material deformation measurements. A general procedure of the measurement is described in detail, including the proposed solution to deal with the speckle effect which appears when large deformations are presented. The performance of the proposed sensor has been compared and validated with a commercial contact LVDT sensor showing measurement differences below 20μm (2.9%).